This invention relates to certain new high modulus silicones, toughened epoxy thermoset resin systems made from those silicones, and composites that are prepared from the new toughened epoxy resin systems.
Epoxy resins, one of the most widely used engineering resins are well-known for their use in composites which utilize high strength fibers. Articles can be manufactured from epoxy resins which are lighter in weight than the same articles manufactured from metals yet retain equivalent strength and stiffness. To date, epoxy composites have been relatively brittle due to the brittleness of the matrix resin. This brittleness restricts the wider application of certain composites because of the obvious problems associated therewith.
This problem has been attacked in a variety of ways, for example, Rowe, E. H. and Siebert, A. R., Mod. Plast. 47, 110 (1970 and McGarry, F. J., Proc. Roy. Soc. Lond. A., 319, 59 (1970) have used linear polybutadiene-polyacrylonitrile copolymers to toughen epoxy resins. Such an approach works to toughen the laminates from such modified epoxy resins, but there is a drastic loss of hot-wet properties.
Jabloner, et al., in U.S. Pat. No. 4,656,207, issued on Apr. 7, 1987, discloses the use of amino terminated polysulfones to toughen epoxy resins. These polysulfones are much more effective toughening agents than polybutadiene-polyacrylonitrile copolymers. These polysulfones also increase the toughness of the laminates made from the epoxy resins. However, these polysulfones must be used at high levels, at least forty weight percent or more, so that the viscosity of mixed epoxy resins is increased.
Organofunctional polysiloxanes have been used in combination with epoxy resins, but not in the toughening sense. For example, Hirose, et al, in European Patent Publication 0 293,733 A2 disclose a curable polymer composition. More particularly, they disclose a curable composition comprising an epoxy resin and an organic elastomeric polymer having in the molecule, at least one silicon-containing group which is crosslinkable through formation of a siloxane bond through a silicon-containing reactive group. This material is easily formulated into a one pack type composition and curable even at room temperature and affords a cured product having improved mechanical properties such as toughness and strength. Thus, they have provided a copolymer, which is elastomeric in nature, and is not indicated as being useful for toughening epoxy resins in composite applications.
In other polysiloxane combinations with epoxy resins, Yorkgitis and coworkers, Yorkitis, E. M.; Tran, C.; Eiss, Jr., N. S.; Hu, T. Y.; Yilgor, I.; Wilkes, G. L. and McGrath, J. E.; "Siloxane Modifiers for Epoxy Resins" in Adv. Chem. Ser. 208 (Rubber Modified Thermoset Resins) 137 to 161, (1984), have used essentially linear aminoalkyl endblocked random copolymers of diphenyl-dimethyl-siloxanes and random copolymers of methyltrifluoropropyl-dimethylsiloxanes to toughen epoxy resins. These materials have been found by Yorkgitis et al to toughen about as effectively as the commercial polybutadiene-polyacrylonitrile copolymers described supra, but these materials also cause a drastic loss of hot-wet properties of the composite.
Yorkgitis et al have correctly stated that "A toughened material, by definition, features improvements in fracture resistance without substantial loss of mechanical strength or modulus." Yorkgitis et al then illustrate that the above-mentioned oligomeric polysiloxanes only slightly influenced the flexural modulus of the base epoxy resin, stating "As expected, the flexural modulus does decrease as rubber content increases. The decrease is less severe as either TFP (trifluoropropylmethylpolysiloxane) or DP (dimethylpolysiloxane) content increases."
From what is disclosed in the prior art, it thus appears that essentially linear or essentially elastomeric organofunctional polysiloxanes are known, but they have not been used effectively to toughen epoxy resins for use in composite applications.
Therefore, it was unexpected that hard, brittle organofunctional silicone resins would toughen a hard, glassy, brittle, thermoset epoxy resin. In other words, the inventors herein believe that this is the first disclosure of the use of a hard brittle organopolysiloxane resin to toughen a hard brittle epoxy resin.